<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app">
        <button v-for="item in arr" @click="changeItem">{{item}}</button>
    </div>
    <script src="./js/vue.js"></script>
    <script>
        console.log(Array.prototype);
        new Vue({
            el: '#app',
            data: {
                arr: [1, 2, 3]
            },

            methods: {
                changeItem() {
                    this.arr[0] = 4
                    // console.log(this.arr);
                    // this.$set(this.arr, 0, 4)
                }
            }
        })
    </script>
</body>
</html>